Inter Service Dependency Model

In any SOA ecosystem, there are bound to be numerous services. Some of the services in will be core services which will be used by most of the services. If this interdependency is not maintained, it may result in spaghettiof services. To have an ordered SOA architecture, one must rely on proven layered architecture which ensures robustness and independence of non adjacent layers.

To measure inter dependence of services, I have developed a simple but effective model, which I call Inter Service Dependency Model. This model is inspired by DSM (Design Structured Matrix.

In this model, services are viewed from provider and consumer perspective and emphasis is on maintaining layered based architecture principles in SOA ecosystem.



The matrix is divided into three distinct areas. The Solid BLACK region is the demarcation between RED and GREEN. One should fill value in GREEN and RED region cells to illustrate inter dependency of services. This dependency strength generically can be defined as



In a well designed SOA eco system following conditions should exist:

1. Matrix is populated only with Zero and Positive numbers
2. RED region is populated with Zero
3. Black region must not be populated ( pretty obvious)
4. GREEN region should be populated with positive numbers.

Above mentioned conditions still not address adjacent layer dependency only. To address that one should be able to figure out structure of GREENs and may also club services into layers.

Reference:

1. http://www.dsmweb.org/
2. http://ausweb.scu.edu.au/aw08/papers/edited/mehboob/paper.html
3. http://www.stsc.hill.af.mil/crosstalk/2005/11/0511SangalWaldman.html
4. http://www.lattix.com/technology/whatisdsm.php
5. http://en.wikipedia.org/wiki/Dependency_Structure_Matrix

Composite Services Complexity Measure

While evaluating a service complexity, I developed few of the measure. One of the measures is Complexity Measure which considers depth of service or we can say composition depth of service.

For this measure composite services were divided into four types:

1. Sequential Composite Services
2. Co-ordination Composite Services
3. Recursive Composite Services
4. Complex Composite Service



So Complexity Measure for:

Sequential Composite Service:

Composite Service Depth, CSD = Distance to the farthest service in sequence

Co-ordination Composite Service:

Composite Service Depth, CSD = Arm Count

Recursive Composite Service:

Composite Service Depth, CSD = (Optimistic recursion count + Pessimist recursion count)/2

Complex Composite Service:

Composite Service Depth, CSD = Sum of (Composite Service Depth in terms of Sequential, Recursion and Co-ordination manner)

For a good implementation CSD should not be more than 3 to 5. But in reality, most of the time consumer of a service is not aware of complete service tree.

Recursive composite service must be avoided.

As a good practice, each service should publish its CSD count.